After its successful encounter with Pluto, the New Horizons spacecraft was sent to a smaller object in the Kuiper Belt. As it rocketed past, the spacecraft captured images of a small world made up of two very different lobes, with features scientists found a little confusing. But details would have to wait, as the combination of distance and energy budget meant sending much of New Horizons’ data to Earth was a slow process.
The wait for that data is now over, as the high-resolution images are now available, and scientists have used it to try and understand the formation and structure of what is now known as Arrokoth (named for the Powhatan word for “heaven”). easier to understand. † While the data doesn’t answer every question we have about Arrokoth, it does give us some very good ideas about how such a strange structure might have formed.
Can’t stay long
New Horizons was the fastest probe as launched from Earth (others have since picked up more speed thanks to gravity), and Arrokoth is very small, meaning the probe had to get quite close before it could be imaged in detail. That left a narrow window for data collection during the flyby, but papers released today indicate how narrow it was. Just two days before the closest pass, Arrokoth still appeared as a single pixel in New Horizons’ cameras. It only got bigger than 10 pixels half a day earlier. So the vast majority of data comes from a window that is only 12 hours wide.
Still, the cameras aboard the New Horizons at the time captured images examining Arrokoth’s composition, and they were able to resolve features as small as 33 meters across the surface. The articles that followed describe the structure of the body, model its history, and make some conjectures about its chemistry.
Arrokoth comes from an area of the solar system called the Kuiper Belt, specifically from an area beyond the gravitational pull of Neptune, the outermost major planet. There was enough material in this region to form icy bodies, but it was spread so thinly that the bodies appear to have remained small, without interacting often enough to form larger planets. Neptune’s influence spread some objects in the Kuiper belt further inward, where collisions with other bodies were more likely and the influence of the sun was stronger. But Arrokoth is currently circling past the point where that would likely happen.
If true, it means the object is likely composed of material that has remained largely unchanged since the formation of the solar system. And, by all the clues, it’s true. Assessments of the crater density on Arrokoth’s surface correspond to an age of four billion years, that of the solar system itself. And the surface has the red color typical of other objects from this region of the Kuiper Belt, suggesting that the surface has not undergone significant chemical change.
The red color appears to come from a complicated mix of longer-chain hydrocarbons collectively called tholins. These are built by chemical reactions between shorter molecules that are powered by exposure to radiation. In Arrokoth’s case, those shorter molecules appear to contain methanol, a one-carbon alcohol and the only individual chemical clearly identified in the New Horizons data. Methanol could have been formed by chemical reactions between methane and water, but there is only weak evidence of the presence of water on Arrokoth and no clear signature of methane. It’s possible — even likely, given what we know about the Kuiper Belt and other objects in it — that they’re below the surface, but that’s not confirmed by this flyby.
Whatever it’s made of, Arrokoth isn’t very dense and probably resembles comets in this regard. If it is less than half as dense as a typical comet, it spins fast enough to disintegrate. Too much closer, and the two lobes would have crushed each other more if they came together.
How two became one
One of the things that calls for an explanation is Arrokoth’s unusual shape. It appears to be a so-called ‘contact binary’, meaning it is formed by two objects gently knocked against each other. But in this case, the objects themselves seemed a bit squashed, which also had to explain their flattened, elongated shape.
One of the main results of the flyby was the generation of two consecutive images from slightly different perspectives, allowing for a stereoscopic view of Arrokoth. The 3D reconstruction created from that rendering indicates that the two lobes are not as flattened as they originally appeared. This level of flattening can be explained by each object’s spin, and the more rounded shape means that each part’s spin only needs to be slightly higher than Arrokoth’s current spin to create the right degree of flatness.
Modeling the kind of collisions that could bring two separate bodies together indicated that any approach speed of more than about five meters per second would result in some rupture of the two bodies, rather than the neat bilobed structure we see. This suggests that the two bodies must have formed close together, from the same collapsing cloud of material. It’s unlikely that anything else offers this kind of gradual approach speed.
But even a slow approach, like the two experienced objects, would have needed something to bleed out their original momentum. So the researchers considered several items that could have done this. But many of the easy options just don’t work. Arrokoth is simply too far from the sun — more than 40 times the distance from Earth to the sun — for light to have a significant effect on the motion of the bodies. Collisions have not been frequent enough to take away enough of the momentum.
What the researchers were left with is the gas that originally orbited the sun early in the history of the solar system. While the sun’s energy dissipated most of this gas, it would have been present when the two bodies were originally formed and, more importantly, orbited the sun more slowly. This would have caused friction to the two bodies that made up Arrokoth, allowing them to approach slowly enough to fuse without crushing either of them at the point where they first made contact.
No rings, big crater
The largest feature on Arrokoth is a crater nicknamed “Maryland” after the location of the New Horizons control center. Maryland lies on the smaller of the two lobes and is about four miles wide and at least half a mile deep. The otherwise rounded perimeter is interrupted by a spur that extends into the crater; how it was formed is not clear. There are plenty of smaller depressions that look like craters, but none of them are more than a mile wide.
Craters usually mean that material was blasted from the surface of a small body like Arrokoth, so the researchers looked for their remains: small moons or rings, which have been found on other small objects in the solar system. But there were no signs of moons, and if a ring is present, it is incredibly scarce.
There are many further details that have already been explored – three articles mean a lot of text and additional data. But the release of the data also means that people studying other processes in the Solar System and objects in the Kuiper Belt will rethink their objects of interest in light of what we now know about Arrokoth. And the publications on that will probably keep coming for another ten years or more.
Science, 2020. DOI: 10.1126/science.aay3705, 10.1126/science.aay6620, 10.1126/science.aay3999 (About DOIs).